A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 5 - Getting a Handle on Hydrology

Forming a Stage-Discharge Relationship

Each time you take a flow measurement, you should take a gage reading. On a sheet of
paper or in a computer file, keep a record of each flow measurement you take and the
corresponding staff gage reading. Once you have enough data, you simply plot these two
variables on a graph and draw or compute the resulting curve.

Draw a graph with an x-axis and y-axis. The x-axis, the horizontal line, will be the
streamflow measurement. The y-axis, the vertical line, will be the staff gage reading.
Place a dot on the graph where each streamflow and corresponding staff gage measurement
intersect. Draw a smooth, curved line between the points. Now you have a stage-discharge
relationship. From now on, you can simply take the gage reading and estimate the stream
flow from your prediction curve.

As convenient as a stage-discharge relationship is, it still needs to be supported by
real data. The more data points you use to develop your graph, the better. The graph is
accurate only for the stream flows that fall within the data range you used to create the
graph. For example, if all your measurements were taken during June through September when
stream flows were low, the graph could not be used to predict high flows in December. Be
sure to collect data during a wide range of flow conditions. In general, if you have about
four data sets from the low-flow period and four from the high-flow period, you can
comfortably prepare the graph. Make periodic checks of the discharge curve, especially
after periods of flooding. Recalibrate the curve if the periodic checks indicate the
relationship has changed. Eventually, natural changes in the stream bottom will result in
a change in the relationship between flow and gage height.

The importance of pollutant loading calculations was described in Chapter Three.
Loading is a simple function of concentration and flow. Loading can be reported in a
number of different units and can be calculated as shown in the table.